This invention relates to a control system for operating a reformed gas generator, in which liquid fuel is converted with primary air and, optionally, a gas containing bound oxygen, into a reformed gas, and an internal combustion engine connected thereto, in which the reformed gas is burned with secondary air.
A method for operating such a system in which, under steady state conditions, the supply of liquid fuel and total air, and the ratio of the primary air stream to the secondary air stream are regulated to values suited for the steady state conditions, is known, for instance, from the German Offenlegungsschrift 23 06 026. This method has the advantage that fuels low in noxious substances (e.g., "straight-run" gasoline or other raw distillates which are produced in refineries in the production of gasoline, which have no additives of lead compounds or aromatics and are therefore not suitable for the operation of modern internal combustion engines, for instance, in motor vehicles, because of their relatively low octane number) can be used for the operation of internal combustion engines. Such liquid fuels are converted in the reformed gas generator, through partial oxidation, into a reformed gas which has a high octane number and burns in the internal combustion engine, developing very little nitrogen oxide, partially burned hydrocarbons, aromatics and other harmful substances.
In the Offenlegungsschrift mentioned above, it is explained that an exhaust gas low in harmful substances is produced only if the ratio, hereinafter referred to as the total air number .lambda..sub.13, of the quantity of air which is actually supplied overall to the reformed gas generator and to the internal combustion engine, to that quantity of air which is required for stoichiometric combustion of the fuel fed in, is larger than 1. This is equivalent to saying that the air number in the combustion of the reformed gas, i.e., the ratio of the quantity of air fed to the internal combustion engine to the quantity of air required for the stoichiometric combustion of the reformed gas, is larger than 1, or that the combustion is hyperstoichiometric.
Since, however, part of the calorific value is lost in the partial oxidation of the liquid fuel in the reformed gas generator, energy saving operation of the internal combustion engine is possible only when excess primary air is not supplied to the reformed gas generator. On the other hand, a certain amount of primary air supply is necessary to maintain the necessary conversion temperature and to avoid the formation of soot in the reformed gas generator. The primary air number, called .lambda..sub.12 hereinafter, is therefore advantageously between 0.05 and 0.2. The primary air number indicates, in the case of partial oxidation of fuel with air, the ratio of the quantity of primary air fed to the gas generator to the quantity of air which would be required for stoichiometric combustion of the converted liquid fuel. The partial oxidation can also be carried out endothermically by means of gases containing the oxygen in bound form. The primary air number then indicates how much air would have to be supplied to the fuel to obtain a reformed gas of the same gross composition. Overall, the primary air stream, the secondary air stream and, if applicable, the exhaust gas return, must be regulated under all operating conditions in such a manner that they are in definite relationships to the fuel supplied.
To this end the above-mentioned Offenlegungsschrift 23 06 026 proposes controlling, on the one hand, the fuel supply substantially in dependence on the position of the gas pedal and the engine speed, and, on the other hand, controlling a throttle, which is arranged in the suction line between the mouth of the reformed gas line and the inlet of the internal combustion engine, according to the gas pedal position. The suction underpressure present upstream from the throttle serves to: first, draw in the secondary air through the suction line, and secondly, to draw primary air into the reformed gas generator and to draw the reformed gas generated from the reformed gas generator into the intake line. An automatic throttle valve which is adapted to the flow resistance of the reformed gas generator and insures that primary air and secondary air are drawn in approximately in constant ratio is arranged in the secondary air feed. If exhaust gas from the internal combustion engine is returned into the reformed gas generator, a suitable exhaust gas metering valve insures that part of the primary air is displaced by exhaust gas without change of the primary air number.
Such apparatus does permit, under steady state operating conditions, the advantageous respective values for the air numbers to be obtained, but rapid load changes are not attainable. Rather, it has been found that a sudden increase of the engine output can be achieved only if pre-evaporated liquid fuel is introduced into the reformed gas generator from a heated supply vessel, or if the liquid fuel and the primary air are heated by an additional heating system. It is not sufficient for this purpose to heat the fuel and the primary air by heat exchange with the reformed gas produced and/or the exhaust gas alone. The above-mentioned control, furthermore, is relatively sluggish and requires careful matching to the flow conditions in the feeds, the reformed gas generator and the heat exchangers. Adaptation to the respective characteristics of the system including the reformed gas generator and internal combustion engine, which could lead to optimum operation as to emission of harmful substances, power output and fuel utilization, requires a considerable amount of technical means. If the system is designed for one mode of operation, later interventive changes are hardly possible.